The present invention relates to a method for actuating a friction clutch, in particular for a vehicle drivetrain.
In motor vehicle drivetrains, friction clutches are often used as separating clutches between a drive unit and a transmission such as a multi-step transmission. The present invention relates in particular to this type of friction clutch, though may also relate to friction clutches such as synchronizing clutches in a transmission.
The progressive automation of motor vehicle drivetrains and the increased demands for fast gearshifts on the one hand and for high comfort on the other hand require sophisticated actuating methods.
It must be taken into consideration here that such friction clutches are often integrated into a transmission housing, and it is consequently difficult to measure certain states of the friction clutch.
Document DE 108 009 094 B4 discloses a method for actuating a clutch, wherein instead of a clutch pressure, a line pressure is measured at a measurement location connected via a transmission path to the hydraulic actuator for actuating the friction clutch, wherein the transmission path contains a hydraulic resistance. To improve the actuation of the friction clutch, it is proposed in said document to pass the measured actual signal through a transfer element with delay characteristics before said signal is transmitted to a regulator.
In said type of actuating method, it is consequently not necessary to measure the pressure in a hydraulic actuator.
Furthermore, the clutch position, which can be set equal to the position of a hydraulic actuator, is of particular significance from a regulation aspect. This is because, in the case of friction clutches of the type mentioned in the introduction, superordinate regulation strategies are often based on the so-called engagement point or kiss point, which corresponds to a position of the clutch or actuator at which the friction clutch begins “to grip”. Here, the engagement point often corresponds to a state in which the friction clutch can transmit a certain very low torque.
The clutch position and the actuator position are however also variables which are difficult to measure.
Document DE 100 46 106 C1 discloses a method for regulating an automatic motor vehicle clutch, in which method the clutch position is measured and a regulating error is determined from a nominal value and from the measured actual value of the clutch position, and a regulating signal is calculated from said regulating error. In said document, to actuate a hydraulic actuator, an electrically actuable fluid source is used which has an electromagnetically actuated valve, by means of the coil current of which a volume flow of hydraulic fluid is controlled, wherein the volume flow builds up a pressure in the hydraulic actuator and thereby defines the position of the clutch. To compensate for non-linearities in the regulating path, a regulation-oriented model is established which is formed by the valve, the actuator and the clutch. An inverse path model is derived from said model. The volume flow is calculated by means of a sliding regulation algorithm. Said volume flow is input into the inverse path model and in this way the valve coil current required to generate the volume flow is calculated.
A similar method, in which an oil volume flow is calculated from a nominal position and an actual position and, in addition, a pilot controller calculates from the nominal position a further oil volume flow and outputs the latter as an output signal of the pilot controller, is known from DE 101 37 581 C1.
In the latter two documents, however, like before, it is assumed that the actual position of the clutch or an actual position of the actuator is measured and taken into consideration in the conventional way to form a regulating error.